There is known a drive system for a vehicle, which includes a differential mechanism operable to distribute an output of an engine to a first electric motor and its output shaft, and a second electric motor disposed between the output shaft of the differential mechanism and a drive wheel of the vehicle. JP-2003-301731A discloses an example of such a vehicular drive system, which is a hybrid vehicle drive system. In this hybrid vehicle drive system, the differential mechanism is constituted by a planetary gear set, for example, and a major portion of a drive force of the engine is mechanically transmitted to the drive wheels through the differential function of the differential mechanism, while the rest of the drive force is electrically transmitted from the first electric motor to the second electric motor, through an electric path therebetween, so that the differential mechanism functions as a transmission the speed ratio of which is continuously variable, for example, as an electrically controlled continuously variable transmission, thereby making it possible to drive the vehicle under the control of a control device, with the engine kept in an optimum operating state with an improved fuel economy.
Generally, a continuously variable transmission is known as a transmission which permits an improved fuel economy of the vehicle, while on the other hand a gear type transmission such as a step-variable automatic transmission is known as a transmission having a high power transmitting efficiency. However, there is not available any power transmitting mechanism having the advantages of those two types of transmission. For example, the hybrid vehicle drive system disclosed in the above-identified publication JP-2003-301731A includes the electric path for transmitting an electric energy from the first electric motor to the second electric motor, namely, a power transmitting path for transmitting a portion of the vehicle drive force as an electric energy, so that the first electric motor is required to be large-sized to meet a need for an increased output of the engine, and the second electric motor driven by the electric energy generated by the first electric motor is also required to be accordingly large-sized, whereby the overall size of the hybrid vehicle drive system tends to be large-sized. It is also noted that a portion of the output of the engine is once converted into an electric energy which is subsequently converted into a mechanical energy to be transmitted to the drive wheels, whereby the fuel economy of the vehicle may possibly be deteriorated under some running condition of the vehicle, for instance, during a high-speed running of the vehicle. Where the above-described differential mechanism is a transmission the speed ratio of which is electrically variable, for example, a continuously variable transmission so-called an “electric CVT”, the vehicular drive system suffers from a similar problem.
In the hybrid vehicle drive system described above, it is well known to provide a step-variable transmission in a power transmitting path between the output member of the differential mechanism (electrically controlled continuously variable transmission) and the vehicle drive wheel, for the purpose of reducing the required capacity of the second electric motor when a high vehicle drive torque is required, for thereby reducing the size of the second electric motor.
As an example of the step-variable transmission, there is well known a step-variable automatic transmission which performs clutch-to-clutch shifting actions each effected by a releasing action of a coupling device and an engaging action of another coupling device. Generally, such a clutch-to-clutch shifting action of this type of step-variable automatic transmission is effected according to a so-called “overlap control” in which overlapping of an engaging torque of the coupling device in the releasing action and an engaging torque of the coupling device in the engaging action is controlled. For instance, the overlap control is implemented to adjust the engaging pressures of the coupling devices in their releasing and engaging actions, and the timings of the releasing and engaging actions, so that an amount of a racing rise (hereinafter referred to as “racing amount”) of the engine speed (input speed of the step-variable automatic transmission) is controlled to a desired value, for reducing the shifting shock and improving the shifting action as felt by the vehicle operator. Thus, the overlap control requires the racing rise of the engine speed (hereinafter referred to as “racing”) during each clutch-to-clutch shifting action, so that the clutch-to-clutch shifting action is effected with the racing of the engine speed, that is, in an insufficient-overlapping state of the engaging pressures of the two coupling devices.
In the drive system including the two transmission mechanisms in the form of the differential mechanism (electrically controlled continuously variable transmission) and the step-variable automatic transmission, however, a shifting action of the step-variable transmission may cause a shifting action of the differential mechanism in response to the shifting action of the step-variable transmission, since the overall speed ratio of the drive system is determined by the speed ratios of the two transmission mechanisms. Accordingly, the drive system including the two transmission mechanisms may require a more complicated control to effect the clutch-to-clutch shifting action, than a drive system including only the step-variable automatic transmission.
When the drive system is operated as a continuously variable transmission as a whole, for example, the clutch-to-clutch shifting action of the step-variable automatic transmission which results in a stepping or non-continuous change of its speed ratio is accompanied with a shifting action of the differential mechanism to change its speed ratio in the direction opposite to that of the step-variable automatic transmission, in the inertia phase of the clutch-to-clutch shifting action in which the input speed of the step-variable automatic transmission changes in synchronization with the progress of the clutch-to-clutch shifting action. As a result, the overall speed ratio of the drive system is changed continuously in the process of the clutch-to-clutch. Thus, it is necessary to restrict a change of the engine speed in the process of the clutch-to-clutch shifting action. At the same time, the clutch-to-clutch shifting action is effected in a state of racing rise of the input speed of the step-variable automatic transmission for reducing the shifting shock. This racing rise of the input speed of the step-variable automatic transmission makes it difficult to concurrently control the shifting action of the continuously variable transmission portion so as to restrict the change of the engine speed in synchronization with the clutch-to-clutch shifting action, so that there is a risk of generation of a considerable shifting shock.
The present invention was made in view of the background art described above. It is therefore an object of this invention to provide a control apparatus for a vehicular drive system including a differential mechanism operable to perform a differential function for distributing an output of an engine to a first electric motor and its output shaft, a second electric motor disposed in a power transmitting path between the differential mechanism and a drive wheel of a vehicle, and a step-variable transmission constituting a part of the power transmitting path and operable to perform a clutch-to-clutch shifting action, the control apparatus permitting reduction of the required size or an improvement of the fuel economy of the vehicular drive system, and reduction of the shifting shock of the step-variable transmission upon the clutch-to-clutch shifting action.